French police are probing the encrypted messaging app, Telegram, after ISIS terrorists used it to post messages prior to Tuesday’s premeditated murder of a Catholic priest in Normandy.

Investigators are trying to obtain more information about 200 members of a closed chat group on the smartphone application, which was used by at least one of the two killers, Adel Kermiche and Abdel Malik Nabil Petitjean, both aged 19, to discuss the murder with other suspected IS-affiliated members.

According to audio messages and chat logs featuring Kermiche, uncovered by French magazine L’Express, he used the application to post a message a week prior to the attack (19 July). It said: “You take a knife, you go in a church, you cause carnage… you cut two or three heads and there you are, it’s done.”

Kermiche, who was previously imprisoned in France between May 2015 and March this year for attempting to travel to Syria, was released on 22 March and forced to wear an electronic tag – used by police to trace his whereabouts during specific times of the day.

Based on analysis of the Telegram messages by L’Express, Kermiche told the group he had been “given ideas” by a mysterious character only referred to as a “sheikh” he met while in prison. He also spoke about wanting to create a terrorist cell in the country and create a group called Sharia4Belgium.

In other messages, Kermiche showed disdain at other commenters in the group who reportedly branded him an “armchair jihadi”. On the day of the attack, 26 July, he left one final message: “Download what is about to happen and share it en masse!!!!!!” There were no further updates.

Kermiche and Petitjean were both shot dead by police marksmen as they exited the Sainte Thérèse church in Saint-Étienne-du-Rouvray, Northern France, after slitting the throat of 86-year old Father Jacques Hamel, wounding another victim and taking hostages during a mass ceremony.

As previously reported, the pair had pledged allegiance to Isis (Daesh) in a video recording before the attack. “You will suffer what our brothers and sisters are suffering,” the footage boasted. “We are going to destroy this country, we will raise the flag and we will elevate the word of Allah.”

“You will suffer what our brothers and sisters are suffering,” the footage boasted. “We are going to destroy this country, we will raise the flag and we will elevate the word of Allah.”

Detectives have yet to fully ascertain how the pair made contact. However, it has emerged both were – to some degree – already known to security services. On Sunday, 31 July, a cousin of Petitjean appeared before a judge after being arrested in connection with the case.

It is alleged the 30-year-old man, named only as Farid K, was aware his cousin would commit a terrorist attack, although not the place or date that it would occur. In a statement, the French prosecutor said the suspect “was fully aware of his cousin’s imminent violent action”.

A separate man, described by police as a Syrian refugee, was arrested in the subsequent operation, but has since been released. Meanwhile, a minor detained on suspicion of being involved in the incident will not be prosecuted but may face charges relating to ‘materials’ found on his computer, reported Sky News.

Telegram has long been associated with supporters and propagandists of the Islamic State. One investigation carried out by IBTimes UK found members of group chats discussing everything from hacking campaigns to military training tactics to real-time news updates on deaths, beheadings and murders.